Conventionally, there is disclosed a technique by which a pipe body, for example, a prism pipe body is made by bending a metal plate, such as in Japanese Patent Laid-Open No. Hei 11-290940.
In Japanese Patent Laid-Open No. Hei 11-290940, a prism pipe body is manufactured by using a rectangular metal plate as a material by means of pressing.
The method of manufacturing prism pipe body includes a first bending step, a second bending step, and a re-striking step. In the first bending step, a primary intermediate product is formed of a metal plate. In the second bending step, the primary intermediate product is processed to form a secondary intermediate product. In the re-striking step, the secondary intermediate product is processed to form a prism pipe body as a final product.
In the first bending step, both width direction sides of the metal plate are bent at the right angle in length direction. Accordingly, the primary intermediate product which includes flanges and a bottom plate is formed. The flanges face to each other. The bottom plate connects the flanges to each other.
In the second bending step, a concave surface having a predetermined width is formed on the bottom plate of the primary intermediate product lengthwise, and at the same time, both ends of the concave surface is bent at the right angle to inside. Accordingly, the secondary intermediate product is formed. The secondary intermediate product includes a pair of side walls which facing to each other. The cross section of the secondary intermediate product is U shape.
In the re-striking step, edges of a pair of flanges (seam) are contacted together by pressing a pair of side walls of the secondary intermediate product inside. Accordingly, a prism pipe body as a final product is formed.
According to this method of manufacturing the prism pipe body, the concave surface which is formed on bottom plate of secondary intermediate product, has a function to restrict a spring back force generated by pressing the pair of side walls together to inside. Accordingly a prism pipe body with square cross section, in which edges of the flanges closely contact together, can be manufactured only by pressing without welding edges of the flanges.